This invention relates generally to a loudspeaker telephone system and more particularly to a loudspeaker telephone system having improved voice switching and power supply circuitry. To eliminate the inconvenience of lifting the handset from its cradle in a conventional telephone system, the loudspeaker telephone system has been developed. For an effective loudspeaker telephone system, it is necessary that howling effects occurring between the speaker and the microphone should be reduced close to zero. The quality of the caller's voice should be natural and transfer between the sending mode and the receiving mode, should be readily accomplished. It is also desirable that in the lowdspeaker telephone system, there is no cutting of the speech at the initiation and termination portions thereof during transmission. Also, there should be no distortion of the transmission caused by noises entering the system through the lines as well as those noises occurring naturally indoors where the telephone is located.
In order to minimize the above-mentioned howling effect, a variable loss device and ON/OFF switching device are employed in loudspeaker telephone systems of the prior art. However, during simultaneous transmission there is still a sudden change and distortion of the speech level at the initiation and termination portions thereof. As a result of this change and distortion, the clarity of speech is poor during initiation and termination. In Japanese laid-open Publication No. 54-69308 of Satoshyoichi, a prior art switching circuit for an interphone is shown wherein a J-K flip-flop is constructed for controlling the making and breaking of variable switching elements in the speech paths. Thereby, protection is provided against cutting of the speech at the initial and terminating portions thereof. However, in this Japanese application, a problem remained for maintaining the naturalness of the subscriber's voice because the switching elements in the circuits have linear characteristics. Moreover, the prior art was devised for an interphone requiring four lines and thereby has been used only for a short-distance call having a narrow width of impedance variation while maintaining high sending and receiving levels.
What is needed is a loudspeaker telephone system for a two line system which provides naturalness of speech without cutting of the voice at the initiation and termination thereof and which substantially eliminates howling and other noise effects.